pr90 wrote:
A meteor stream is composed of dust particles that have been ejected from a parent comet at a variety of velocities. These particles follow the same orbit as the parent comet, but due to their differing velocities they slowly gain on or fall behind the disintegrating comet until a shroud of dust surrounds the entire cometary orbit. Astronomers have hypothesized that a meteor stream should broaden with time as the dust particles' individual orbits are perturbed by planetary gravitational fields. A recent computer-modeling experiment tested this hypothesis by tracking the influence of planetary gravitation over a projected 5,000-year period on the positions of a group of hypothetical dust particles. In the model, the particles were randomly distributed throughout a computer simulation of the orbit of an actual meteor stream, the Geminid. The researcher found, as expected, that the computer-model stream broadened with time. Conventional theories, however, predicted that the distribution of particles would be increasingly dense toward the center of a meteor stream. Surprisingly, the computer-model meteor stream gradually came to resemble a thick-walled, hollow pipe.
Whenever the Earth passes through a meteor stream, a meteor shower occurs. Moving at a little over 1,500,000 miles per day around its orbit, the Earth would take, on average, just over a day to cross the hollow, computer-model Geminid stream if the stream were 5,000 years old. Two brief periods of peak meteor activity during the shower would be observed, one as the Earth entered the thick-walled "pipe" and one as it exited. There is no reason why the Earth should always pass through the stream's exact center, so the time interval between the two bursts of activity would vary from one year to the next.
Has the predicted twin-peaked activity been observed for the actual yearly Geminid meteor shower? The Geminid data between 1970 and 1979 shows just such a bifurcation, a secondary burst of meteor activity being clearly visible at an average of 19 hours (1,200,000 miles) after the first burst. The time intervals between the bursts suggest the actual Geminid stream is about 3,000 years old.
1.The primary focus of the passage is on which of the following?
(A) Comparing two scientific theories and contrasting the predictions that each would make concerning a natural phenomenon
(B) Describing a new theoretical model and noting that it explains the nature of observations made of a particular natural phenomenon
(C) Evaluating the results of a particular scientific experiment and suggesting further areas for research
(D) Explaining how two different natural phenomena are related and demonstrating a way to measure them
(E) Analyzing recent data derived from observations of an actual phenomenon and constructing a model to explain the data
This question asks you to identify the primary focus of the passage. The best answer is the second choice. The author describes the new theoretical model in the first paragraph; in the final paragraph the author states that the data obtained from actual observations, which are discussed in the second and third paragraphs, is consistent with the new theoretical model.
The first choice is not correct; the computer model confirmed the astronomers' hypothesis that meteor streams broaden with time.
The third choice is also incorrect. Although the model yielded an unexpected result, the passage makes no reference to further areas for research.
The fourth choice incorrectly mentions two natural phenomena. Although the passage does mention two natural phenomena—a meteor stream and planetary gravitation—it focuses on only a single phenomenon, the meteor stream; there is mention that meteor streams are affected by planetary gravitation, but there is no explanation of how they relate, nor any demonstration of how to measure the phenomena themselves.
The last choice is not correct because it reverses the order of events. The model yielded a prediction that was subsequently confirmed by observational data; the model was not constructed to explain the data.
2. According to the passage, which of the following is an accurate statement concerning meteor streams?
(A) Meteor streams and comets start out with similar orbits, but only those of meteor streams are perturbed by planetary gravitation.
(B) Meteor streams grow as dust particles are attracted by the gravitational fields of comets.
(C) Meteor streams are composed of dust particles derived from comets.
(D) Comets may be composed of several kinds of materials, while meteor streams consist only of large dust particles.
(E) Once formed, meteor streams hasten the further disintegration of comets.
3. The author states that the research described in the first paragraph was undertaken in order to
(A) determine the age of an actual meteor stream
(B) identify the various structural features of meteor streams
(C) explore the nature of a particularly interesting meteor stream
(D) test the hypothesis that meteor streams become broader as they age
(E) show that a computer model could help in explaining actual astronomical data
4. Q. It can be inferred from the passage that which of the following would most probably be observed during the Earth's passage through a meteor stream if the conventional theories mentioned in line 18 were correct?
(A) Meteor activity would gradually increase to a single, intense peak, and then gradually decline.
(B) Meteor activity would be steady throughout the period of the meteor shower.
(C) Meteor activity would rise to a peak at the beginning and at the end of the meteor shower.
(D) Random bursts of very high meteor activity would be interspersed with periods of very little activity.
(E) In years in which the Earth passed through only the outer areas of a meteor stream, meteor activity would be absent.
5. According to the passage, why do the dust particles in a meteor stream eventually surround a comet’s original orbit?
(A) They are ejected by the comet at differing velocities.
(B) Their orbits are uncontrolled by planetary gravitational fields.
(C) They become part of the meteor stream at different times.
(D) Their velocity slows over time.
(E) Their ejection velocity is slower than that of the comet.
6. The passage suggests that which of the following is a prediction concerning meteor streams that can be derived from both the conventional theories mentioned in line 18 and the new computer-derived theory?
(A) Dust particles in a meteor stream will usually be distributed evenly throughout any cross section of the stream.
(B) The orbits of most meteor streams should cross the orbit of the Earth at some point and give rise to a meteor shower.
(C) Over time the distribution of dust in a meteor stream will usually become denser at the outside edges of the stream than at the center.
(D) Meteor showers caused by older meteor streams should be, on average, longer in duration than those caused by very young meteor streams.
(E) The individual dust particles in older meteor streams should be, on average, smaller than those that compose younger meteor streams.
7. It can be inferred from the last paragraph of the passage that which of the following must be true of the Earth as it orbits the Sun?
(A) Most meteor streams it encounters are more than 2,000 years old.
(B) When passing through a meteor stream, it usually passes near to the stream’s center.
(C) It crosses the Geminid meteor stream once every year.
(D) It usually takes over a day to cross the actual Geminid meteor stream.
(E) It accounts for most of the gravitational perturbation affecting the Geminid meteor stream.
8. Which of the following is an assumption underlying the last sentence of the passage?
(A) In each of the years between 1970 and 1979, the Earth took exactly 19 hours to cross the Geminid meteor stream.
(B) The comet associated with the Geminid meteor stream has totally disintegrated.
(C) The Geminid meteor stream should continue to exist for at least 5,000 years.
(D) The Geminid meteor stream has not broadened as rapidly as the conventional theories would have predicted.
(E) The computer-model Geminid meteor stream provides an accurate representation of the development of the actual Geminid stream.
Please explain how to answer these questions.
Edit: Added missing questions.
80.
This question asks you to identify the primary focus of the passage.
The best answer is B. The author describes the new theoretical model in the first paragraph; in the
final paragraph the author states that the data obtained from actual observations, which are
discussed in the second and third paragraphs, is consistent with the new theoretical model.
Choice A is not correct; the computer model confirmed the astronomers’ hypothesis that meteor
streams broaden with time, and although the model yielded an unexpected result, the passage does
not contrast the predictions yielded by competing theories. Choice C and D are not correct because
the passage makes no reference to further areas for research, and only a single phenomenon is
described in the passage. And choice E is not correct because it reverses the order of events. The
model yielded a prediction that was subsequently confirmed by observational data, the model was
not constructed to explain the data.
81.
This question asks you to identify an accurate statement about meteor streams. Choice C, the best
answer, restates information about the composition of meteor streams from the first sentence of
the passage.
Choice A is not correct. The passage discusses the influence of planetary gravitation on meteor
streams but says nothing about its influence on the orbits of comets. According to the passage, it is
planetary gravitation, not the gravitational fields of comets, that causes meteor streams to increase
in size, so choice B is not correct. And choice D and E are not correct answers because the passage
says nothing about the composition of comets or the role that meteor streams play in their further
disintegration.
82.
This question asks what the author says about the purpose of the research described in the first
paragraph.
The best answer is D. According to the author, the purpose of the computer-modeling experiment
was to test the hypothesis that meteor streams broaden with time.
Choice A is not correct; although the observational data described in the last paragraph allowed
scientists to estimate the age of the Geminid stream, this data was analyzed to confirm a surprising
prediction made by the computer model. This analysis was not part of the original experiment.
Choice B is also incorrect. Although the experiment yielded a surprising prediction about a
particular feature of meteor streams, the purpose of the experiment was to determine whether
meteor streams broaden with time, not to identify the various structural features of meteor streams.
Choice C is not correct because the experiment was undertaken to test a general hypothesis about
meteor streams. It was not undertaken to explore the nature of any particular meteor stream, and
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the passage never suggests that the actual meteor streams used in the computer model was
“particularly interesting.”
Choice E is not correct. Although the computer model did confirm the astronomers’ hypothesis,
the purpose of the experiment was not to show that such models are useful.
83.
This question asks you to make an inference about what would most probably be observed during
the Earth’s passage through a meteor stream if the conventional theories mentioned in the passage
were correct. According to line 18-20, the conventional theories predicted that the meteor stream
would be most dense at the center. The computer model, one the other hand, predicted that a
meteor stream would come to resemble a thick-walled, hollow pipe (lines 21-22). The passage
states that, if the computer model were correct, two peak periods of meteor activity would be
observed as the Earth passed through the walls of the “pipe” (lines 28-31). According to lines
36-38, observational data confirmed the prediction of the computer model. If, on the other hand,
the conventional theories were correct, it can be inferred that a bifurcation of meteor activity
would not be observed; instead, it can be inferred that scientists would expect to observe a single
peak of meteor activity as the Earth passed through the dense center of the stream. Choice A
identifies this single peak of activity as the most likely observation if the conventional theories
were correct.
Choice B and D are not correct because they describe meteor activity that is either steady or
erratic, neither of which is consistent with the conventional theories. Choice C describes meteor
activity more in line with the bifurcation predicted by the computer model, rather than the single
peak of activity that the conventional theories would suggest. Choice E is incorrect because the
passage says that meteor showers occur whenever the Earth passes through a meteor stream; it
cannot be inferred that either theory would predict otherwise.
84.
This question asks for the reason given in the passage for a characteristic feature of meteor
streams. According to lines 1-7, the dust particles in a meteor stream eventually surround a
comet’s original orbit because of the different velocities at which they are ejected, as stated in
choice A, the best answer.
Choice B is directly contradicted by information in the passage (lines 8-10). The other answer
choices re incorrect because the passage does not say that the dust particles become part of the
meteor stream at different times, or that their velocity slows over time, or that their ejection
velocity is slower than that of the comet.
85.
This question asks you to identify a prediction that can be derived from both the conventional
theories about meteor streams and the new computer-derived model. You must base your answer
on information that is suggested by, but not expressly stated in, the passage.
According to lines 7-8 of the passage, the conventional theories hypothesized that meteor streams
should broaden with time, and the computer simulation confirmed this hypothesis. The passage
also suggests that the time it takes for the Earth to cross a meteor stream (and, by implication, the
duration of the resulting meteor shower) is directly related to the breadth of the stream (lines
23-28). From these pieces of information, which are supported by both the conventional theories
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and the new computer-derived theory, it can be inferred that on average the meteor showers
caused by older (and therefore broader) meteor streams would be longer in duration than those
caused by very young (and therefore narrower) meteor streams, as stated in D, the best answer.
Choice A is incorrect because it contradicts the predictions of both the conventional theories (that
the particles will be most dense at the center of the stream) and the computer model (that the
stream will resemble a thick-walled, hollow pipe). Choice C is also incorrect because it is
inconsistent with the conventional theories that suggested the distribution of dust in a meteor
stream is denser at the center. And choices B and E are incorrect because the theories discussed in
the passage do not suggest anything about the likelihood that the Earth’s orbit will cross that of
any particular meteor stream, nor do they suggest anything about the size of the dust particles that
compose meteor streams.
86.
This question asks you to draw an inference from information in the last paragraph of the passage.
The best answer is C. According to the passage, the Geminid meteor shower occurs yearly;
because meteor showers occur whenever the Earth passes through a meteor stream, one can infer
that the Earth crosses the Geminid stream once every year.
Choice A is incorrect because the passage provides no information from which to generalize about
the age of meteor streams. Choice B, which is directly contradicted by lines 31-32, is also
incorrect. Choice D is incorrect. In lines 25-28, the passage says that the Earth would take just
over a day to cross the stream if the stream were 5,000 years old. However, in lines 38-42 the
passage states that in fact an average of only 19 houses elapsed between the time that the Earth
entered the stream until the time that it exited, leading researchers to conclude that the stream is
only about 3,000 years old. Choice E is incorrect because the passage says only that planetary
gravitational fields perturb the orbits of dust particles in a meteor stream; it does not say that the
effect of the Earth’s gravitation is greater than that of other planets.
87.
This question asks you to identify an assumption underlying the last sentence of the passage. In
this sentence, the author of the passage draws a conclusion about the age of the Geminid stream.
This conclusion is based on two pieces of information. The first is the length of time the Earth
would take to cross the computer-model Geminid stream if the stream were 5,000 years old (lines
24-28). The second is the actual elapsed time between the two peaks of meteor activity predicted
by the computer model (lines 36-40). In concluding from this information that the Geminid stream
is actually only 3,000 years old, the author is assuming the accuracy of the computer model, as
stated in E, the best answer.
Choice A is incorrect because the passage says that the time the Earth takes to cross the stream
would vary from year to year (lines 32-34) and that 19 hours was the average time, not the exact
time, observed from 1970 to 1979 (lines 36-40). Choices B and C are incorrect because the
passage does not suggest anything about the current state of the comet associated with the
Geminid stream or about the expected longevity of the stream. Choices D is incorrect because the
computer model is said to confirm the broadening predicted by the conventional theories; the fact
that the model projected the positions of the particles in the stream over a 5,000-year period does
not suggest that researchers expected the stream to be older (and therefore broader) than it turned
out to be.
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